FIG. 14 shows an embodiment of a DC-DC converter similar to the one disclosed, for example, in JP-A-2001-224171 (pages 7 and 8 and FIG. 1). As illustrated, a series circuit, including a metal oxide semiconductor field-effect transistor (MOSFET) 1 and a MOSFET 2, is connected in parallel with a direct current (DC) power supply 3. A series circuit, including a capacitor 4, an inductor 5, and a primary winding P1 of a transformer 6, is connected in parallel with the MOSFET 2. Diodes 7, 8 and a capacitor 9 that rectify and smooth a positive or negative voltage developed at secondary windings S1 and S2 of a transformer 6 are connected to the secondary windings. A series circuit, including the capacitor 4, inductor 5, and primary winding P1 of the transformer 6 is connected in parallel with the MOSFET 1. The inductor 5, which is connected in series with the primary winding P1, can be substituted with the leakage inductance of the transformer 6.
A feedback signal is transferred to a control circuit 11 via a voltage detection circuit 10 to keep a voltage V0 across the capacitor 9 constant. The control circuit 11 includes a comparison circuit that compares the feedback signal with a predetermined value, and a pulse generator that generates a gate pulse to be fed to the MOSFET 1 based on the results of the comparison.
In FIG. 14, the control circuit 11 and a drive circuit 12 alternately turns on and off the MOSFETs 1 and 2 at a duty cycle of 50%. An output voltage is regulated by varying the driving frequency for the MOSFETs 1 and 2. An integrated circuit (IC) capable of withstanding a high voltage and having a level shift capability can be used as a means for transferring a signal, with which the MOSFET 2 on a high-potential side (hereinafter high-side) is turned on or off, from the control circuit 11 to the drive circuit 12. However, since the IC capable of withstanding a high voltage is relatively expensive, the use of the IC increases the overall cost of a switching power supply device.
FIG. 15 shows an embodiment similar to the one disclosed, for example, in JP-A-2002-209381 (page 4 and FIG. 1) (refer to column 4, lines 10-23, and FIG. 1 of corresponding U.S. Pat. No. 6,483,722), that can reduce the cost. In the illustrated embodiment, the transformer 6 has a tertiary winding P2, and the high-side MOSFET 2 is driven via the drive circuit 12 according to a positive or negative voltage developed at the tertiary winding P2. In JP-A-2002-209381, the drive circuit 12 includes a resistor. The low-potential side (hereinafter low-side) MOSFET 1 is driven so that it is turned on after the MOSFET 2 is turned off. Thus, the two MOSFETs are alternately turned on and off. Owing to the foregoing configuration, the control circuit 11 can employ an inexpensive IC capable of withstanding a low voltage. Consequently, the cost of the switching power supply device can be reduced.
In the circuitry shown in FIG. 15, the on time of the MOSFET 2 is determined with a voltage developed at the tertiary winding P2 of the transformer 6. The on times of the MOSFET 1 and MOSFET 2 are usually different from each other. Consequently, current flowing through the MOSFETs 1 and 2 and current flowing through the rectification diodes 7 and 8 are unbalanced. This, however, undesirably increases the losses occurring in the MOSFETs and rectification diodes.
In particular, when a main AC voltage is rectified and smoothed as a DC supply voltage to be fed from the DC power supply 3, since the AC voltage varies within a certain range, the ratio of the maximum value of the DC supply voltage to the minimum value thereof gets almost doubled. The higher the DC supply voltage is, the more remarkable the unbalance between the currents flowing through the MOSFETs and rectification diodes becomes.
Accordingly, there remains a need for an improved switching power supply device, or more particularly, to a technology for designing a half-bridge direct current-to-direct current (DC-DC) converter, that can resolve the unbalance between the currents flowing through MOSFETs and the currents flowing through rectification diodes so as to thus minimize losses, as well as being compact and low-cost. The present disclosure addresses this need.